This invention relates to double balanced mixing at microwave frequencies.
In many microwave front end circuits, a number of microwave signals received by a system antenna are applied to a mixer, such as a Schottky diode mixer, for down-conversion to intermediate frequency (IF) signals. Along with the fundamental IF frequencies, the mixer typically generates intermodulation products due to the nonlinearity of response of the Schottky diodes. Third order intermodulation products, for example, are quite close in frequency to the fundamental IF frequencies and therefore are difficult to remove by filtering. Ideally, the dynamic range of the mixer (i.e., the maximum received signal power at which mixer is designed to be used) is delimited by the 1 dB compression point. However, the dynamic range is restricted if the third order intercept (IP.sub.3) point, which is the lowest received signal power level at which the power level of a third order product equals the power level of a fundamental IF signal, occurs below the 1 dB compression point.
Maas, in "A GaAs MESFET Mixer with Very Low Intermodulation", IEEE Transaction on Microwave Theory Techniques, Vol. MTT-35, No. 4, April, 1987, pp 425-29, and "A GaAs MESFET Balanced Mixer with a Very Low Intermodulation", IEEE MTT-S International Microwave Symposium Digest, Vol. II, 1987, pp 895-98, shows single ended and single balanced mixing using the channel of a common-source, GaAs MESFET (metal electrode semiconductor field effect transistor) as the mixing element. The local oscillator (LO) is applied to the gate electrode with a bias voltage and the microwave input signal ("RF") is coupled to the drain electrode. The IF signal is extracted from the drain electrode with a filter. The LO energy leaking across the drain to source terminals of the MESFET is short circuited so that the MESFET operates in its linear (i.e., unsaturated) region, thereby minimizing intermodulation distortion. Thus, a high third order intercept is achieved at microwave operating frequencies in the range of 10 GHz and with bandwidths of about 500 MHz. But because the RF and IF signals both appear at the drain electrode, the RF and IF frequency bands cannot overlap.
Double balanced mixers, on the other hand, internally isolate the RF and IF signals, and thus permit the RF and IF frequency bandwidths to overlap. Conventional double balanced mixers operating in the UHF frequency band (e.g., from 100 MHz to 800 MHz), constructed as a ring of silicon MOSFETs or JFETs, achieve high intermodulation suppression and concomitant high third order intercept. However, relatively high gate-to-drain and gate-to-source capacitances typically limit the upper operating frequency of MOSFET and JFET mixers to below the microwave frequency range (i.e., the frequency range from about 1 GHz to about 30 GHz).